1. Field of the Invention
The present invention relates to a surface treatment agent which is possible to make a metal surface rust-proofing and to improve the coating film adhesion, and in particular to a metal surface treatment agent suitable for use with aluminum products such as pre-coated aluminum sheets.
2. Description of the Related Art
Metal surfaces have been treated conventionally to improve the corrosion resistance of the metal surfaces, with a number of/types of metal surface treatment agents being used. Of the various types of metal surface treatments, chromate treatment using a compound containing chromic acid is commonly used, since chromate treatment gives metal an excellent corrosion resistance and also exhibits good properties in terms of adhesion to paints.
However, it has been pointed out that the chromium used in chromate treatment causes environmental pollution, and hence in recent years alternative metal surface treatment methods and surface treatment agents have been developed. Materials of such surface treatment agents include tannic acid, organophosphorus compounds, silane type coatings, and surfactants, as disclosed in Hyomen Gijutsu (‘Surface Technology’), 49 (3), 221 (1998). Moreover, a surface treatment agent obtained by copolymerizing an unsaturated carboxylic acid (Japanese Patent Publication No. H5-222324) and a surface treatment agent that is a copolymer of a glycidyl-group-containing unsaturated monomer and and acrylic acid ester (Japanese Patent Publication No. H3-192166) are known. These materials all use ank acrylic type resin and, to achieve sufficient corrosion resistability, the coating film must be made thick. Moreover, the adhesion of these materials to various metals such as iron and aluminum is not always adequate, and in a wet environment the adhesion may drop markedly resulting in the coating film peeling off. Epoxy resin type materials, on the other hand, give improved adhesion to the substrate. For example, a water-soluble coating composition comprising water and an alkali-neutralized reaction product of the reaction between a phosphoric acid containing P-OH bonds, an epoxy resin and a glycidyl (meth)acrylate (Japanese Patent Publication No. H5-148447), and an epoxy resin composition comprising a polyglycidyl compound and a phosphoric acid ester containing P-OH bonds obtained from a phosphoric acid and a monoglycidyl ether or ester compound (Japanes,e Patent Publication No. H9-176285), have been proposed. However, although these materials give good adhesion, it is necessary to make the coating film thick to improve the corrosion resistability.
In contrast with the above, in Japanese Patent Publication No. 2001-39927, the present inventors disclosed a novel tricarbonyl compound, a novel tricarbonyl group-containg acrylic copolymer, and a metal surface treatment agent using the same, as a surface treatment agent that adheres strongly to a metal surface and gives excellent corrosion resistance and corrosion resistability, even in the case of a thin film. Furthermore, in Japanese Patent Publication No. 2001-316835, the present inventors disclosed a metal surface treatment agent in which an epoxy ester reaction mixture between a phosphoric acid type compound and an epox yesin is associated with a silane compound or a titanium compound.
However, although the metal surface treatment agents according to the prior art described above give excellent corrosion resistability and are suited to applications in which this treatment is the finishing process (for example an automobile evaporator), application to so-called pre-coated aluminum sheets, i.e. aluminum plates further coated with a polyester, a fluororesin, an epoxy resin or the like, is difficult. In the case of aluminum plates used in automobile evaporators, corrosion resistability is required first of all, and coating film adhesion is not required that much. With pre-coated aluminum sheets, on the other hand, the surface is painted, and hence various properties are required of the aluminum plate after the painting. Specifically, not to mention the corrosion resistability of a painted aluminum plate, the user may use the aluminum plate after bending, and hence coating film adhesion, flexibility and ease of bending are important. In addition, with surface treatment agents applied to pre-coated aluminum sheets, there are calls to move from organic solvents to water-based solvents.
Furthermore, depending on the usage environment, surface treatment agents applied to pre-coated aluminum sheets may be required to give the pre-coated aluminum sheet surface acid resistance.